The present invention relates to desalting/concentrating apparatus including a reverse osmosis membrane. The apparatus pressurizes feed fluid under high pressure and supplies it to the membrane, and the fluid is separated into a permeate fraction which permeates through the membrane and a concentrate fraction which does not permeate the membrane.
U.S. Pat. No. 4,432,876 to Keefer shows a prior art reverse osmosis apparatus. With particular reference to FIG. 2 of the patent, a reciprocating feed pump 60 includes a pumping chamber 26.2 and an expansion chamber 37.2 which contains a portion of piston rod 63 and is therefore smaller in cross-sectional area than the pumping chamber 26.2. The pumping chamber 26.2 connects through check valves 32.2 and 33.2 respectively with a feed fluid conduit 28.2 and a membrane pressure vessel 17 (FIG. 1). The expansion chamber 37.2 connects through directional valves 42.2 and 44.2 respectively with the membrane pressure vessel and an exhaust conduit.
A pumping stroke forces the feed fluid out of the pumping chamber 26.2 through the membrane pressure vessel into the expansion chamber 37.2 under high pressure because of the difference in volume between these chambers, thereby separating a permeate fluid fraction 13.2. An induction stroke sucks a feed fluid into the pumping chamber 26.2, and exhausts a concentrate fluid fraction 14.2.
The directional valves 42.2 and 44.2 are timed from a reciprocable drive 48 (FIG. 1) to open after initiation of a pumping or exhaust stroke respectively so that the valve actuation lags behind the piston stroke.
Upon initiation of a pumping or induction stroke, there is a sudden pressure rise respectively in the pumping or expansion chamber as the appropriate directional valve has not yet opened. This pressure rise is reduced by a volume exchange means 64 provided between the outfeed and return conduits 30.2 and 38.2.
The provision of volume exchanger 64 increases the cost and size of the apparatus. In addition, the exchanger 64 interrupts the pump operation for an instant at each end of the pump strokes, thus reducing the concentration capacity.
Japanese Patent Appln. 62-99271 filed on Apr. 22, 1987( (Provisional Pub. 63-264105 laid open on Nov. 1, 1988) and Appln. 62-311227 filed on Dec. 9, 1987 (both filed by Sasakura Engineering Co., Ltd.) show other reverse osmosis apparatus as prior art.
In Provisional Pub. 63-264105, the piston rod 9 of a reciprocating pump holds a nut 12 fixed thereto and engaging a screw shaft 11, which is rotated alternately in opposite directions by an electric motor 13 to reciprocate the piston 8.
In Appln. 62-311227the reciprocating pump pistons 11 and 12 are fixed to a screw shaft 17 engaged with a nut 18, which is likewise rotated alternately by a motor 19.
In these apparatus, for the timing of a directional control valve to shift at each end of the piston strokes, the motor is required to temporally interrupt rotation at each end of the piston strokes, in order to shift the valve, or otherwise the pump is required to lower its reciprocating speed. This reduces the concentrating capacity. Also, the motor frequently repeats alternate rotation, and the wear between the screw and the nut is great, thereby lowering the durability of the apparatus.
It is an object of the present invention to provide a reverse osmosis apparatus, including a directional control valve which can be shifted accurately at each end of the pump piston strokes.